Devices for calculating and displaying the load supported by cranes, derricks, and the like, have long been used as operator aids in preventing unstable conditions or the overstressing of structural elements in the crane boom. This capability is particularly important in mobile cranes of the type having telescopingly extendable booms which can be slewed through the whole or part of a circle during normal operation. By comparing the load indication of an operating aid with the load rating tables supplied by the crane manufacturer for a specific crane and operating configuration, an operator can determine the relative stability of the crane. Typically, two methods of determining the load supported by a crane have been employed.
The first method involves the direct measurement of the actual weight of the load by devices such as tensiometers, strain gauges, and the like.
The second method involves the calculation of the total effective hook load, which is determined by first measuring geometrical parameters of the crane boom such as length and angle and then calculating the total turning moment of the boom and load about the boom pivot pin. By dividing the total turning moment by the horizontal radius of the load from the pivot pin, the total effective load can be calculated.
With both methods, the actual load or total effective load can thus be determined and displayed to the operator who, upon referral to the load rating tables, can determine the amount of crane lifting capacity remaining at any given time.
The measurement of crane geometrical parameters such as slewing angle, luffing angle, and boom length are often accomplished with potentiometers which produce a variable resistance as a function of the parameter being monitored. Typical of these arrangements is the device described in the invention to Greb, et. al., U.S. Pat. No. 3,489,294 which discloses a load limit control for hoisting equipment including a cable payout scheme driving a potentiometer to measure boom length.
Such systems while generally satisfactory have certain shortcomings and disadvantages. First, they are inherently analog devices which are relatively difficult to interface with a digital load moment computer, requiring A/D conversion. Additionally, because variable resistors are contacting type devices, they are subject to mechanical wear and long term instability. When used in a sensor employing a pendulum configuration such as one which measures boom angle, friction from the contacting type device results in inaccuracies as well as dither wear. Additionally, prior art analog systems may not detect an electrical failure such as a partial short or an open circuit which could result in giving the crane operator a dangerously misleading load indication.